Variable permeability tuning device



Dec. 8,1942. R. L. HARVEY 2,304,751 VARIABLE PERMEABILI'IY TUNING DEVICE Filed May 3 1, 1941 Patented Dec. 8, 1942 2,304,761 r VARIABLE ruamzammrr TUNINGDEVICE Robert L. Harvey, Collingswood, N. J assignor to Radio Corporation or America, a corporation of Delaware 7 Application May 31, 1941, Serial No.

1 Claims. (01. 171-242) This invention relates to variable permeability tuning devices for use in radio, television, and other systems for the communication of intelligence and has special reference to the provision of an improved form of inductance and inductance changing core means therefor.

It is an object of the present invention to provide an extremely compact variable permeability tuning device and one nevertheless characterized by an extended tuning or inductance variation range.

Still another object of the present invention is to provide an arrangement which makes possible thefuse of an inherently low permeability (high Q) material, such as magnetite, for covering a substantially wide tuning range heretofore attainable only with the use of molded comminuted pure iron cores.

Another and important object of the invention is to provide an improved variable permeability tuning device wherein the magnetic flux is confined within or closely adjacent the coil whereby undesired coupling effects upon the associated circuits and apparatus is minimized.

The foregoing objects are achieved in accordance with the invention by the provision of a coil which has the electrical closed path characteristics of a toroid but which has the physical shape of a hollow helical toroid to permit the entry therein and withdrawal therefrom of a helically shaped inductance changing core having the same "closed path characteristics as the coil.

In the accompanying drawing Fig. 1 is a plan view of a helical coil form and core constructed in accordance with the Principle of the invention,

Fig. 2 is a side elevational view looking in the direction of the arrows in Fig. l, and showing the device in circuit,

Fig. 3 is a side elevational view of the core of the device of Fig. 1 and Fig. 4 is an enlarged end view of the tuner of Figs. 1-3.

Fig. 5 is a plan view of a tuning mechanism embodying the invention and showing one form of drive for the magnetic core,

Fig. 6 is an elevational view illustrating another form of driving mechanism.

In the drawing wherein like reference characters represent the same or corresponding parts in all figures, I designates a. hollow coil form in the shape of a helix, 3 designates an inductance changing core slidably mounted with the form and 5 designates an inductance coil wound about the outer surface of said form. If the coil 5 is of the self-supporting type, the form I may be omitted.

The coil form I may be constituted of a synthetic resinous material, such for example, as polystyrene, in which case it may originally have been in the form of a straight tubewhich was later softened by the application of heat and bent in the indicated shape with the coil 5 already in position thereon.

The inductance changing core 3 may be constituted in its entirety of a ferrbmagnetic compound such, for example, as the one described in U. 3. Patent 1,274,952 to Speed, though in some cases it may be constituted in its entirety of copper, or it may comprise a multisection unitary structure. By way of example, referringnow to Fig. 6 the inductance changing core 3 may comprise three continuous sections 3A, 3B and 30, wherein the central section B is constituted of a ferromagnetic compound, an end section C of copper or thelike (to insure a maximum change in inductance as it is screwed into or out of the coil 5) and the opposite end section A may be composed of an insulating substance through which a driving force may be applied by means of a control knob l, or the like.

As shown more clearly in Figs. 2 and 3, the inductance changing core 3 preferably extends slightly more than 360 about its axis of generation, so that its ends overlap, and ensure an air-gap of reduced reluctance for the magnetic flux. For optimum results, the coil 5 should extend substantially 360 about its axis of generation so that it in effect provides a substantially magnetically continuous enclosed or confined path for the flux. The separation between the adjacent ends of the toroidal assembly is advantageous in that it reduces the overall distributed capacity, thereby increasing the Q, or factor of merit, compared to that obtainable with a true toroidal construction.

In the above connection it may be recorded that inductance variation ratio of 11 to 1 has been achieved in a permeability tuning device wherein the coil and its magnetic core, which in this example was magnetite, were of the pattern above described. This makes possible covering at least a 3 :l frequency coverage. This approaches the maximum tuning range, theoretically possible where magnetite (the permeability of which is 13), is employed, and is substantially twice that usually obtainable in a stright core and coil assembly. Using pure powdered iron in the molded magnetic core gives a very substantially greater.change in the inductance ratio since iron has a much greater theoretical permeability. However. attention is called to the fact that magnetite is more desirable for high frequency work (e. g., up to 60 megacycles) since its losses are low compared to those of pure iron.

Any suitable means may be provided for eii'ecting relative movement between the coil l and its core 3. Thus, referring to Fig. 4 in molding the core a groove may be provided on its outer surface to accommodate a cord 9 (Fig. through which a driving force may be applied through a suitable pulley arrangement II, in which case the cord may comprise a support for a pointer H to be read in connection with a suitable scale l5.

Other modifications oi the invention will suggest themselves to those skilled in the art. It is to be understood therefore that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the spirit of the appended claims.

What is claimed is:

1. In combination, a coil wound substantially in the form of a helical toroid, an inductance changing core of helical form adapted to be entered into said coil, said helical core extending more than 360 degrees about its axis of generation whereby the air-gap between the ends thereof comprises a path of relatively low reluctance 10 stituted at least in part of a non-magnetic metal.

4. In combination, a coil wound in the form of a helical toroid and providing a substantially enclosed path for the magnetic flux of said coil, and a helical shaped inductance changing core adapted to be entered into said coil and providing a substantially continuous path for said flux, and means for moving said core into and out oi said coil.

5. The invention as set forth in claim 4 and wherein said last mentioned means comprises a helical insulating'extension on said core through which a driving force may be applied to said core.

6. The inveintion as set forth in claim 4 and wherein a helical extension of insulating material on said core lies within said coil for supporting and guiding said core.

7. The invention as set forth in claim 1 and wherein said helical core comprises magnetite.

ROBERT L HARVEY. 

